The invention relates to strong and/or super acid catalysts, which have many potential uses. In particular, the invention relates to preparation of such catalysts by fluorinating polymers which contain pendant acid groups, such as sulfonic acid groups or their alkali metal salts.
Acid catalysts are employed in many commercial applications, such as oil refining and chemical processes, where acid catalysts such as BF.sub.3, HF, AlBr.sub.3, HBr, AlCl.sub.3, HCl, H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, zeolites, and ion exchange resins are found. In recent years, disposal of acids and hazards associated with their use have drawn the attention of the public and government regulators and consequently, there is an incentive to limit their use. To do so will require that substitutes be found which are efficient and which create fewer environmental problems.
One potentially useful material is acidic ion exchange resins, which are already employed in alkylation of aromatics and production of MTBE (methyl tertiary butyl ether). Typical of such resins are sulfonated macro reticular polystyrene/divinyl benzene resins such as Amberlyst 15 (Rohm and Haas), Dowex M-32 (Dow Chemical) and Lewatit (Bayer AG).
It is necessary to define the strength of acids in terms other than the familiar pH scale when one is discussing catalysts which may be characterized as strong or superacid catalysts. For purposes of the present invention the Hammett acidity scale will be used (1) Umansky, B., J. Engelhardt, W.K. Hall. J. Catal., 1991, 127, 128-140; (2) Buttersack, C., H. Widdecke, J. Klein. J. Mol. Catal., 1986, 35, 77-99.) Acids on this scale have a value from 0 to -28. When the value is below -12 (the strength of 100% H.sub.2 SO.sub.4), the acid will be considered a superacid. For comparison, Amberlyst 15 has a Hammett acidity of -2, Nafion NR50 has a value of -6.5 to -11, 100% HF a value of -11, BF.sub.3 /HF -15 and HF/SbF.sub.3 -28 (Olah, G.A., G.K.S. Prakash, J. Sommer. "Superacids." New York: J. Wiley, 1985).
Fluorination of polymers has been suggested for various purposes. Dixon et al. in U.S. Pat. No. 4,020,223 disclose treatment of polyolefin and polyacrylonitrile fibers with fluorine in the presence of small amounts of oxygen to increase the amount of fluorination.
Fluorination of various polymers in the absence of oxygen but using a cold plasma was disclosed by Anand et al. in U.S. Pat. No. 4,264,750.
Functionalized polymers containing pendant ester, carboxylic acid, acid halide, or acid anhydride groups were fluorinated by fluorine-inert gas mixtures by Lagow as shown in U.S. Pat. No. 4,076,916.
Boultinghouse, in U.S. Pat. No. 4,296,151 discloses the fluorination of plastic articles with fluorine in an inert gas to make the surfaces more wettable with water. The polymers were based on hydrocarbons such as olefins, dienes, and vinyl-substituted aromatics.
McGinniss et al. in U.S. Pat. No. 4,491,653 examined the surface-fluorination of polymers and determined that the oxygen content should be restricted and that partial fluorination of the surface to produce --CHF-- groups was desirable.
Chiao in U.S. Pat. No. 4,828,585 described the fluorination of gas separation membranes using fluorine and sulfur dioxide gases.
In U.S. 4,593,050 Cohen et al. disclose the use of ultraviolet light to assist in the fluorination of polymer surfaces using various fluorinated species, including fluorinated hydrocarbons.
Bliefert, et al. in U.S. 4,536,266 disclosed surface fluorination of various macromolecular materials by using elemental fluorine dissolved in liquids including halogenated hydrocarbons where the fluorine concentration is only 0.5.times.10.sup.-3 to 1.times.10.sup.-2 mol/l.
In U.S. Pat. No. 4,522,952 and DE 302345 SC2 Klein et al. disclosed a process for the fluorination of polymers and in particular those which contain sulfonic acid groups, --SO.sub.3 H, particularly --SO.sub.2 F, such as sulfonated crosslinked styrene/divinylbenzene. Klein et al. state that the group --SO.sub.3 Na is undesirable since it produces products which tend to decompose. The fluorination reaction is carried out with the fluorine in the gas phase, beginning with a low concentration and gradually increasing until pure fluorine gas is used. The process is carried out at below ambient to ambient temperatures and without solvents. Klein et al. state their preference for replacing at least 90% of the hydrogen atoms in the polymer with fluorine atoms. The fluorinated polymer is said to have high catalytic activity for the alkylation of phenol with isobutane or benzene with propene.
In H. Brown, Dissertation, Technical Univ., Braunsweig, Germany, 1985; J. U. Schluter, Dissertation, Technical Univ., Braunsweig, Germany, 1987, the fluorination of ion exchange resins was discussed. They reported that potassium- and cesium-exchanged resins were superior to those containing the acid group --SO.sub.3 H and suggested adding alkali metal fluorides to increase the fluorination rate. The catalytic activity was found to reach a maximum at a degree of fluorination between 45 to 75%, with much reduced activity seen at either higher or lower levels of fluorination. The fluorination was carried out with fluorine in an inert gas and dilution of the fluorine was said to be important to avoid damage to the polymer matrix resulting in a loss of mechanical strength. The --SO.sub.3 Na groups were found to be fluorinated with the lowest loss of --SO.sub.3 H groups compared with fluorination of --SO.sub.2 F and --SO.sub.2 Cl. Schluter used fluorotrichloromethane (CFC-11) in conjunction with high flow rates of a fluorine/nitrogen gas mixture. The high gas flow rates used indicate that the reaction was primarily a gas-phase reaction by the uniform distribution of fluorine throughout the resin beads. Schluter indicated that gas phase fluorination was preferred. This was also the conclusion in French patent 1,453,455 as reported by Klein et al.
The present inventors have sought and found an improved method for fluorinating porous polymers which is carried out in the presence of an inert diluent in the liquid phase and produces a highly active acid catalyst using less fluorine than the Klein et al. process and which avoids the mechanical degradation of the polymer resulting from a severe fluorination.